Several interrelated studies have investigated the development of mastery motivation. Three of the projects employed the same sample of 75 children to study the interrelationships between cognition, motivation, and the environment at 6, 12, and 30 months, and 6 1/2 years of age, respectively. At each age point, methods were developed that were developmentally appropriate yet conceptually similar to the data collected at the earliest age. A separate study was conducted to investigate the expression of mastery motivation in Down syndrome infants. The current focus of the 6, 12, and 30 month studies is directed toward explicating the relationship between mastery motivation, competence, and social abilities. The results indicate that in 6- and 12-month-old infants, manipulative activities directed toward success with objects represent an important dimension of their behavioral repertoire. Moreover, these early manifestations of the motivation to be competent are an important aspect of later cognitive abilities. While these findings extend through to the early school-age years, it appears that the quality of mother-infant attachment serves to mediate these relationships. With regard to the development of mastery motivation and social competence in Down syndrome (DS) infants, it appears that in comparison to a cognitively normative sample, DS infants are not as adept at integrating the object-related and socially-directed domains into ongoing streams of behavior.